Quality factor (video compression)

The quality factor (qf) of encoded video streams (a.k.a. frame quality) is the amount of bits for each pixel in a given video. It should be noted that the quality factor is not a true estimation of visual quality, as different visual elements may require more or less bits to appear the same quality.

Calculation

The quality factor can be calculated by the formula:

$Q_f = \frac{1000 \cdot b}{v \cdot h \cdot f}$

where Q_f is the quality factor, b is the bit rate (expressed in kbit/s), v is the vertical resolution of the frame, h is the horizontal resolution of the frame, and f is the frame rate (expressed in frames per second).

As an example, consider a 90 minutes video with a resolution of 640x360 pixels and a frame rate of 23.976 frames per second (frame/s) to be encoded. The video is supposed to fit onto a 700 MiB CD. If you reserve 100 MiB for the audio stream, this will leave you with 600 MiB for the video stream, which equals a video bandwidth of

$b = \frac {600 \cdot 1024 \cdot 1024 \cdot 8} {90 \cdot 60} = 932,067.(5)\ bit/s = 932.067(5)\ kbit/s$

At a frame rate of 23.976 frame/s the quality factor equals

$Q_f = \frac {1000 \cdot 932.067(5)} { 640 \cdot 360 \cdot 23.976 } = 0.1687$ bits per pixel.

Accordingly you can use the formula to calculate the bit rate needed to produce an encode of a given Q_f.